# Homework Help: Thermodynamics Hw help, piston designing

1. Nov 17, 2012

### Nilkc

1. The problem statement, all variables and given/known data
Have a design project in thermodynamics. The problem reads,

Someone has suggested that the device shown in the figure below be used to improve the maximum force F against the spring, which has a spring constant of k. This is accomplished by changing the temperature of the liquid-vapor mixture in the container. You are to design such a device to close sun-blocking window-shutters that require a maximum force of 0.5 lbf. The piston must move 6 inches to close those shutters completely. You elect to use R-134a as the working fluid and arrange the liquid-vapor mixture container such that he temperature changes from 70 degrees F when shaded from the sun to 100 degrees F when exposed to the full sun. Select the sizes of the various components in this system to do this task. Also select the necessary spring constant and the amount of R-134a to be used.

2. Relevant equations

Force=pressure*Area
Force=k*x

3. The attempt at a solution
All of my work and all the info I have been using is in the attachments. I have been working it on a whiteboard at school. I can only load 3 pics. I continued on and found that the natural length of the spring is 9.707 inches with a k value of 0.031833 Lbf/in.

I am assuming that at 100 degrees F that the system is in equilibrium and the force to keep the windows closed is 0.5 lbf. The picture also shows the piston is closed off, so I am assuming that I don't need to worry about atmospheric pressure. I can't see where I am going wrong, but the piston diameter seems to small. Is this right or am I reading the original problem wrong.

#### Attached Files:

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2. Nov 19, 2012

### Nilkc

Got it figured out.

3. May 21, 2013

### pjh0118

I have assignment as what you did. So can you please give me your answer? so I can get some ideas. Please Help me.

4. Nov 25, 2013

### violinak

Hi Nilkc, i have the same project and i'm having trouble with that, could you give me a hint?? hELP

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